I recently purchased an XPS-15 and the TB-16 dock. The supplied Thunderbolt cord comes out of the dock on the left side, but the associated port on the laptop is also on the left side. This results in the dock being behind the laptop precluding easy access to the USB ports on the front of the dock. Alternatively, I've tried facing the laptop away from me (which works because I'm using a dual monitor setup with nothing on the laptop monitor. Near the end of the manual for the TB-16 it shows how to remove the cord but Dell apparently doesn't make a longer cord. Research on Amazon for an extension Thunderbolt cable turned up one, but it is not recognized as a Dell cable. Thoughts or workarounds?
I go into gory detail in my thread about Thunderbolt and USB-C cables, but essentially under the current limits of our regular passive cable technology, 0.5 m / 18" is the longest cable that's able to pass Thunderbolt 3 40 Gbps, which is what the TB16 dock requires. Longer passive cables only pass 20 Gbps, which won't be enough for the dock, and they also may only be rated to carry 60W of power, which is not enough for what the TB16 can deliver. There are active cables that can support 40 Gbps out to 2 m / 6 ft (although only 1 m / 3 ft if you also need more than 60W of power), but those cables are $50-85 apiece, and unfortunately you wouldn't be able to use them in the TB16 anyway because the connector on the dock side is proprietary, not a standard Thunderbolt connector, and I don't believe Dell has made an active version of this cable with their proprietary connector.
Extension cables often introduce more signal degradation than a single cable of the same total length, for what it's worth, because there's signal dropoff when crossing connectors, so even in scenarios where they would work (e.g. USB/Ethernet cables), I tend to recommend against them because you'll likely experience lower bandwidth.
In terms of solving the physical placement issue, you could perhaps perch your laptop on top of the dock, or maybe get some sort of riser stand tall enough for the dock to fit under and wide enough to create a surface for you laptop to rest on securely. Otherwise, you may want to consider getting a USB hub that you can locate more freely on the desk.